The present invention relates to a bolt fastening method and apparatus.
A bolt fastening method (torque method) in which the bolt fastening management is conducted by utilizing the proportional relationship between the fastening torque T and the fastening shaft force (in the bolt elasticity area) is known. While this torque method is superior in work efficiency, there may occur a large variation in the fastening shaft force according to the coefficient of friction between the screw face and the seat face of the bolt (see FIG. 11). Accordingly, another bolt fastening method (torque+angle method), which is shown in US Patent Application Publication No. 2006/0218768, has been used, in which the bolt is fastened up to the predetermined snug torque Ts (a fastening torque at a start point for an effective and substantial fastening) and then the bolt is further rotated (fastened) by a specified angle θk from the standard bolt angle corresponding to the snug torque Ts, thereby restraining improper influence of the above-described coefficient of friction (between the screw face and the seat face). According to this bolt fastening method, since the bolt fastening management is conducted only with the bolt fastening angle (rotational angle) θk after the bolt fastening up to the snug torque Ts, a fastening shaft force F based on the bolt fastening angle θk, which is different from the torque method, may receive little influence of the coefficient of friction during the bolt fastening.
In the above-described bolt fastening method (torque+angle method), however, there may occur the variation in the fastening shaft force according to the coefficient of friction (between the screw face and the seat face) in the fastening area before reaching the snug torque Ts, which may improperly affect the fastening shaft force F of the fastening after the fastening point of the snug torque Ts (see FIG. 12). In this case, while it may be considered to set the snug torque Ts at a very low torque for the purpose of restraining the variation of the fastening shaft force (fastening angle) at the snug torque Ts, there occurs necessity to cope with restrictions of fastening conditions (for example, there is a problem in that it may be recognized by mistake to have reached the snug torque due to dusts being stuck or the like). Thereby, the above-described setting would not be sufficient.